The present invention relates generally to sensing devices, and more specifically to improved heater controllers for silicon, micro-machined thermal sensing devices and to thermal sensing devices having the improved heater controllers.
Silicon, micro-machined thermal sensing devices that employ the principle of free convection heat transfer of a hot air bubble in an enclosed chamber are well-known to the art. Referring to FIG. 1, such devices 10, typically, include a chamber 12 that has been micro-machined or etched into a silicon substrate 40. A heater resistor 15 is structured and arranged across the chamber 12 and electrically coupled to a power supply 14 via a controller or modulator 70. X-axis 22 and 28 and y-axis thermocouple pairs 24 and 26 are disposed on opposing sides of the heater resistor 15, to measure temperature changes in two axes.
In U.S. Pat. No. 6,795,752, which is co-owned by MEMSIC, Inc. of Andover, Mass., the assignee of the present invention, a control circuit is disclosed for an integrated convective accelerometer device. During conditions of zero acceleration, the temperature profile about the heater resistor 15 is symmetrical such that the thermocouple pairs in the x-axis 22, 28 and in the y-axis 24, 26 sense the same temperature and, therefore, provide the same output voltage. Acceleration applied along the thermocouple-heater-thermocouple axis causes disturbance of the temperature profile due to free convection heat transfer, thereby causing an asymmetrical temperature profile.
The asymmetrical temperature profile is sensed by the thermocouple pairs, to provide output voltages that differ and a differential output voltage that is proportional to the applied acceleration. The differential output voltage typically requires signal conditioning to interface with the electronics of a particular application. Such signal conditioning is implemented using external electronic components and/or circuitry combined on the same substrate as the convective accelerometer.
FIG. 2 shows an illustrative control circuit for regulating heater power in accordance with the prior art. As shown in FIG. 2, a cavity 12 is micro-machined or etched into a silicon substrate 40. A heater resistor 15 is structured and arranged across the cavity 12, e.g., using a suspended bridge. Thermocouples 20 and 30 are structured and arranged on opposite sides of the heater resistor 15.
Voltages at the respective positive terminals, V2+ and V2−, of the thermocouples 20 and 30 are applied as input to a chopping, stabilized instrument amplifying circuit (“instrument amplifier” or “chopper”) 60. Voltages at the respective negative terminals, V1, of the thermocouples 20 and 30 are commonly applied to a heater power adjusting circuit 90, e.g., a voltage divider. In the closed-loop control system, heater power is set by the resistive tapping point of the heater power adjusting circuit 90.
In the instrument amplifier 60, each of the positive terminal voltages, V2+ and V2−, is input into the positive terminal of respective operational amplifiers (“opamps”) 62 and 64. The common-mode voltage, Vcom, of the thermocouple outputs is equal to the average of the positive terminal voltages, V2+ and V2− and, furthermore, is proportional to heater power. Hence, common-mode voltage, Vcom, of the instrument amplifier 60 can be used to provide a measure of heater power to a heater closed loop control circuit 100.
For example, as shown in FIG. 2, the common-mode voltage, Vcom, can be input into the negative terminal of an integrating operational amplifier circuit (“integrator”) 85 while the positive terminal is electrically coupled to a constant voltage source, Vbs. So configured, the integrator 85 serves as an error amplifier to drive the common-mode voltage, Vcom, to a fixed voltage, i.e., Vbs, which, in FIG. 2, is 1 Volt (V).
Problematically, start-up time associated with the control loop associated with U.S. Pat. No. 6,795,752 is relatively slow. Indeed, as shown in FIG. 3, the thermal sensing circuit includes a heating circuit 21, a flowing fluid 23, a thermal couple circuit 25, and a heater control circuit 27, which are spatially-disposed relatively close to one another. As a result of this spatial proximity, in order to make the feedback system stable, the frequency pole of the heater control circuit 27 must be the dominant pole. However, because the frequency pole of the heater control circuit 27 has a very low frequency, system response time is relatively slow. In short, the dilemma facing designers requires a compromise between system stability and a faster start-up time.
It would therefore be desirable to have an improved silicon, micro-machined thermal sensing device that provides a faster start-up time while maintaining system stability within acceptable limits.